This invention relates to a method of making golf ball molds and, more particularly, to a method of making imperfectly spherical golf ball mold halves and golf balls therefrom.
Golf ball molds are used in the production of golf balls of both the wound and solid types. Normally upper and lower mold halves are used to make one golf ball and each of the mold halves is generally the size of one half of the golf ball. When the two mold halves are mated, they will form a generally spherical interior with an equator at approximately the middle of the golf ball.
The golf ball mold halves can be made using a master pattern or hob that approximately corresponds to the dimensions of a finished, unpainted golf ball. This hob can then be used to impress a plurality of mold blanks to make a plurality of golf ball mold halves. An example of this process is disclosed in U.S. Pat. No. 3,831,423 to Brown, et. al.
In the past, the hob used to make some golf ball molds had an outer surface that was a section of a sphere. This surface is indented with dimples or other texture that is typically applied to a golf ball""s surface. The golf ball mold that is made from this hob thus has an interior surface in the shape of a sphere, with dimple or other texture forming projections extending inwardly from the spherical surface. Surprisingly, the golf ball that is formed by this type of mold is usually not perfectly spherical. Rather, it usually has a first diameter when measured generally perpendicular to an equator plane or pole-to-pole, a similar second diameter when measured generally parallel to the equator plane, and a third somewhat smaller diameter when measured in orientations roughly half way between the first two orientations. Since these diameters are not equal, the ball is not perfectly spherical. This is most notable in a compression molding process, but occurs with other processes as well.
The United States Golf Association (USGA), the organization that sets the rules of golf in the United States, has instituted a rule that a ball must not be manufactured to have properties which differ from those of a spherically symmetrical ball. The non-spherical shape of golf balls created using spherical hobs creates difficulty in complying with this symmetry rule. Various methods can be used to compensate for the non-spherical shape of a ball. For example, U.S. Pat. No. 5,908,359 to Shimosaka, et al. discloses a method of arranging and dimensioning the dimples on the surface of a golf ball to compensate aerodynamically for the effect of the non-spherical shape.
Therefore, it is desired to have a method of making golf ball mold halves so that a golf ball produced from such a method is more spherical.
The present invention is directed to an improved method of making golf ball mold halves. The mold halves are made by impressing a golf ball hob into a mold blank and applying pressure to form an impression of the hob in the mold blank. The golf ball hob has an arcuate surface that is defined by at least two arcs having different center points.
In one embodiment, the arcuate surface has a profile from its equator edge to its pole, and the profile is composed of at least two different arcs. In another embodiment, the profile of the arcuate surface is comprised of three different arcs. Preferably, each of the three arcs extends through an angle of 30 degrees.
According to another aspect of the invention, the radii of the arcs near the pole and near the equator are larger than the radius of the arc in-between. Preferably, the radius of the arc nearest the pole and the radius of the arc nearest the equator are equal.
According to one embodiment, each arc is continuously arranged to each adjacent arc. Preferably, the adjacent arcs are in a tangential arrangement where they meet each other.
The present invention also includes a method of making golf balls. The balls are made using a pair of mold halves. Each half includes a cavity with an arcuate surface defined by at least two arcs with different center points. A golf ball subassembly is placed into a golf ball mold cavity defined by the pair of mold halves, and a cover material is disposed between the subassembly and the golf ball mold halves.
In one embodiment, the subassembly is a solid, one-piece core. In another embodiment, the subassembly is a solid polybutadiene center surrounded by at least one intermediate layer. Preferably, the at least one intermediate layer is an ionomer resin having an acid content of less than 20 weight percent.
In another embodiment, the subassembly is a two-piece core having a solid center and an intermediate layer wound around it. Alternatively, the center is liquid filled.
The cover material can be a thermoset polyurethane, a thermoplastic urethane, a balata, a thermoplastic elastomer, a thermoset urethane, a castable urethane, reaction injection molded urethane, or a material that is at least partially an ionomer. Preferably, the cover is castable polyurethane.